(a) Field of the Invention
The present invention relates to a system and method for estimating output current of a direct current-direct current (DC-DC) converter used for environmental-friendly vehicles such as a fuel cell vehicle, an electric vehicle, a plug-in electric vehicle, and a hybrid vehicle. More particularly, the present invention relates to a DC-DC converter that can perform an overcurrent protection function and a power derating function by directly applying an output current map to the output voltage of a current transformer based on the output voltage of the DC-DC converter without using an output current sensor, and by estimating output current more accurately using an output current sensor by minimizing an error due to the output voltage of the DC-DC converter through correction, and a method for estimating output current of a DC-DC voltage.
(b) Description of the Related Art
In general, it is known in the art that output current Iout is required to be measured in real time to perform output overcurrent protection and power derating functions in a DC-DC converter. The measured output current Iout is transmitted to a CPU (Central Processing Unit) that is in charge of a voltage control and protection function of the DC-DC converter and the CPU protects power parts in the DC-DC converter while performing overcurrent protection and output limit functions based on the measured output current.
Two known methods of measuring the output current Iout of the DC-DC converter, are as follows. One method includes directly measuring output current using a current sensor mounted at the output terminal of the DC-DC converter. This method achieves more precise current sensing with an error at the level or ±1 A, but the use of the current sensor increase the overall material cost. Another method includes indirectly estimating the output current Iout, using a CT (Current Transformer) used for control and protection in DC-DC converters in the related art. Although this method does not use a current sensor, as shown in FIG. 1, in a DC-DC converter 10, the output current Iout may be expressed by Equation 1 below showing the relationship between input power Pin, output power Pout, and efficiency η of the DC-DC converter 10, and from which, the output current is expressed by the relational expression of input voltage Vin, input current Iin, efficiency η, and output voltage Vout, as in Equation 2.
                    η        =                                            P              out                                      P              in                                =                                                    V                out                            ·                              I                out                                                                    V                in                            ·                              I                in                                                                        Equation        ⁢                                  ⁢        1                                          I          out                =                                            V              in                        ·                          I              in                        ·            η                                V            out                                              Equation        ⁢                                  ⁢        2            
In the above equations, the amount of change of the input current Iin can be sensed by a CT and converted into secondary voltage, as shown in FIG. 2, and is converted into new CT output voltage VCT_LPF of 0-5V in an OP-AMP (Operation Amplifier) through an LPF (Low Pass Filter) of a controller 12 of the DC-DC converter 10, and is input to a CPU (e.g., a processor) of the controller 12 of the DC-DC converter 10. Accordingly, the parameters for the amount of change of the input current Iin can be replaced with the CT output voltage and Equation 2 for the method of estimating output voltage, using a CT, described above may be schematized, as shown in FIG. 3. As a result, the output estimation value Io_est can be calculated using Equation 2, schematized into FIG. 3 into a software logic in the CPU of the controller of the DC-DC converter 10, the output overcurrent protection and power derating functions an be performed.
However, according to the method of estimating output current, the measurement errors of the input voltage Vin, input current Iin, output voltage Vout, and efficiency η may be multiplied, causing the errors to be accumulated in the final output current estimation value Io_est, and as show in FIG. 3, an input current map and an efficiency map may change based on the output voltage Vout, causing the estimation error to increase unless error compensation is provided to the output voltage Vout. Further, even when the logic shown in FIG. 3 is implemented in the CPU of the controller of the DC-DC converter, the calculation equations are used excessively and the calculation load of the CPU increases.
As described above, as the estimation error of the output current increases, the function of using the output current value for the overcurrent protection and output limit value is operated abnormally and this may cause unstable control of the DC-DC converter.
The above information disclosed in this section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.